There are a variety of tools for color correction for video and still images, such as color wheels, levels and curves.
Color wheels commonly are used to set hue offsets for pixels within different ranges of luminance values. A pixel that has luminance within a specified range is modified by the hue offset set on a color wheel.
A “levels” display includes a histogram that shows how many pixels an image contains at each brightness level (for combined channels) or how many pixels an image contains at each level for each color component (for individual channels). A user can modify the levels defining the highlights, midtones and shadows in the input image to an output image, thus modifying the histogram to create new pixel data for the output image.
Curves allow a user to manipulate one or more function curves that adjust individual components of pixels of an image, such as red, green, blue components. Additionally, a master curve allows the user to manipulate simultaneously all of the components of an image with a single function curve. The RGB curves allow a user to perform a mathematically nonlinear adjustment to a color component. These curves are particularly useful for color correction because a large number of color problems in an image are nonlinear problems created at the time of acquisition of the image, and these problems are typically introduced in the red, green and blue components of pixels of the image.
As an example, in the SYMPHONY editing system from Avid Technology, Inc., the user interface for RGB curves appears as shown in FIG. 1. In this interface, four curves are displayed, for the red component 10, the green component 12, the blue component 14 and a master 16.
As another example, in the PHOTOSHOP editing system from Adobe Systems, Inc., the user interface for an RGB curve appears as shown in FIG. 2. In this interface, only one curve 20 is shown. This interface includes gradients 22 and 24 adjacent to the two axes of the function curve display. Gradient 22 informs a user of the range of input values for the channel along the horizontal axis. Gradient 24 informs a user of the range of output values for the channel along the vertical axis.